I have carcinosarcoma as well. I too had a CA125 and a pelvic exam every three months, but in addition a CT scan at six months. Unfortunately, the 6 month scan picked up multiple metastatic lesions in my lungs. I've had no pelvic recurrence.
My oncologist believes the pulmonary metastasis that I have experienced is due to the fact that my initial diagnosis was caught at Stage IA with no pelvic metastases. It's more likely I guess that recurrence for you would be pelvic -- and I don't know if a CT scan is as necessary for early detection of recurrence in the pelvis or abdomen, where MMMTs usually recur.
I think you should consider doing everything you can to build your immune system and stay in remission. Reduce stress, get a good eight hours sleep every night and lead as balanced and happy a lifestyle as possible. There is good evidence to consider adopting a vegan diet (though I have modified mine to include fish and goat yogurt.)
Do not let your doctors write you off. My oncologist has had two women with carcinosarcoma recur and then survive beyond five years and be declared cancer free.
I am having a tough time getting to remission since my recurrence but am not giving up hope. After my recurrence, I had a great response to carboplatin and taxol for two cycles, then the tumors in my lungs stopped responding and one actually increased by .4 centimeters. Now I'm on gemzar and taxotere.
I sure hope you have no recurrence!